And if you own two or more horses, your costs increase exponentially. So it's no wonder that a good number of my multiple-horse-owning clients would love to find a saddle that will work for both - or in some cases, all - of their horses.
Is this possible? In some cases, yes. And sometimes - to quote a line from an old Calvin and Hobbes strip - "A good compromise leaves everyone mad."
If you have horses that are all of a similar physical size and type with similar backs and fitting requirements - say, a pair of barrel-bodied Arabs, or a few lean, high-withered off-the-track Thoroughbreds - it's possible. Not optimum, but possible. The general rule of thumb is to fit the widest horse, and use correction pads for the less wide horse(s). This is a situation in which a foam-flocked saddle is the best solution, since the foam will not begin to take on the shape of an individual horse. I know quite a few trainers who specialize in working with a particular type of horse - Thoroughbred eventers or dressage warmbloods - who have just one or two foam-flocked saddles that they use on the horses they work, and it works out quite well.
But say you have two very different horses - let's use the examples I mentioned before. You have Ed, a 14.2 hand Arab with a low wither and a back you could serve dinner on:
You also have Bob, a 16.3 hand OTTB with a "roof" back and a good wither:
These two conformations are so radically different that there's no one saddle that will fit both of them, no matter what kind of corrective pad you use. Ed will need a flat tree (and very possibly a hoop tree, to boot), a shallow rear gusset and perhaps an upswept panel; Bob will need a more curved, standard tree with either a K or trapezius panel and wither gussets, and perhaps a deeper rear guesset. A saddle that would work for Ed would sit down on Bob's withers and bridge, while a saddle that would work for Bob would rock, and perch on Bob's back like a party hat. In this case, two different saddles would be an absolute necessity.